Coating compounds that cure by means of two independent processes are generally known as dual cure systems. The binder components that they contain conventionally have different functional groups, which generally crosslink with one another independently under suitable conditions. Conventional dual cure systems of the prior art have radiation-curing and heat-curing groups, with particularly advantageous properties being obtained with the use of isocyanate and hydroxyl groups as thermally crosslinking functions.
EP-A 0 928 800 describes a dual cure system containing a crosslinking agent that contains both radiation-curing acrylate groups and isocyanate groups that can be heat-cured with suitable binders containing e.g. OH groups. Since NCO and OH groups react with one another even at room temperature, the above coating system can only be handled as a two-component system in which the NCO-containing and NCO-reactive components are mixed together shortly before or during the coating process. The disadvantage of only a very short pot life for these systems can be eliminated by blocking the free NCO groups. The combination of such radiation-curing and heat-curing systems that contain blocked isocyanate groups is described for example in EP-A-126 359, WO-A 01/42329 or U.S. Pat. No. 4,961,960.
With most blocked polyisocyanates of the prior art, the blocking agents they contain are released during the crosslinking reaction and then liberated. This not only has a negative effect on the VOC content of the coating systems, but released blocking agent also remains in the paint film, exerting a negative influence on the coating properties. The scratch resistance and acid resistance of one-component (1C) paint films are thus generally significantly poorer than in two-component (2C) polyurethane paint finishes (e.g. T. Engbert, E. König, E. Jürgens, Farbe & Lack, Curt R. Vincentz Verlag, Hanover October 1995). The release of the blocking agent and its escape from the paint film in gaseous form can also lead to blistering in the paint. Afterburning of the emitted blocking agent may be necessary in some cases.
For particularly low crosslinking temperatures in the range from 90 to 120° C., isocyanates blocked with diethyl malonate have recently been used (e.g. EP-A 0 947 531). In contrast to blocking reactions with e.g. N-heterocyclic compounds such as caprolactam or butanone oxime, in this case not all of the blocking agent is released, leading to an interesterification at the diethyl malonate during curing, with release of ethanol. The disadvantage here, however, is that because of the unstable ester bond such systems are extremely susceptible to the action of acid, such that the possible applications of these products are restricted.